Recently, the SCR type exhaust gas purification apparatus is proposed as the exhaust gas purification apparatus that removes nitrogen oxide (NOx) contained in the exhaust gas of the engine. A reducing catalytic converter included in the exhaust gas purification apparatus is provided in an exhaust gas system, and a reducing agent is injected from a nozzle to the exhaust gas on an upstream side of the reducing catalytic converter, to subject a catalytic reduction reaction of NOx in the exhaust gas and the reducing agent to thereby perform a purification treatment of NOx to a harmless component. Because ammonia having good reactivity with NOx is adopted in the reduction reaction, a urea aqueous solution, an ammonia aqueous solution, or an HC liquid reducing agent (referred to as liquid reducing agent while a precursor thereof is included) is adopted as the reducing agent. The urea aqueous solution and the ammonia aqueous solution are hydrolyzed with exhaust gas heat and water vapor in the exhaust gas to easily generate ammonia.
Recently, the SCR type exhaust gas purification apparatus having the PM collecting function in addition to the NOx removing function is also achieved in accordance with tightening of emission gas regulation. In such cases, it is necessary to further arrange the PM collecting filter to the exhaust gas system in which the reducing catalytic converter and the nozzle which injects the liquid reducing agent into the exhaust gas at the upstream side of the reducing catalytic converter are disposed. Thus, it is requested to achieve space-saving by devising a layout in which the PM collecting filter is further arranged in addition to the nozzle and the reducing catalytic converter. For example, there has been proposed an exhaust gas purification apparatus described in Japanese Laid-Open (kokai) Patent Application Publication No. 2007-040224 (Patent literature 1).
The exhaust gas purification apparatus disclosed in Patent literature 1 includes a first housing in which an oxidation catalytic converter for oxidizing nitrogen monoxide in exhaust gas and the PM collecting filter are contained, and a second housing in which the reducing catalytic converter and an ammonia oxidation catalytic converter for oxidizing excess ammonia are contained, in which the nozzle that injects the liquid reducing agent is provided in a pipe that connects the first housing and the second housing. The first housing and the second housing are disposed while sandwiching a chassis frame therebetween (that is, the first housing and the second housing are transversely disposed), which allows for compact layout of the exhaust gas purification apparatus in a front-back direction of a vehicle (paragraph [0006], [0007], and [0009] of Patent literature 1).
In the exhaust gas purification apparatus disclosed in Patent literature 1, in order to evenly diffuse the liquid reducing agent in the exhaust gas, it is necessary that the pipe between the housings have a certain length. Therefore, the pipe is provided so as to transverse the chassis frame (paragraph [0010] of Patent literature 1). That is, in the exhaust gas purification apparatus, the need for securing the length of the pipe arises in order to evenly diffuse the liquid reducing agent, and therefore, the miniaturization is hardly achieved in the length direction from the upstream to the downstream of the exhaust gas flow, and the restriction that only the layout disclosed in Patent literature 1 can be made is imposed.